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Sleep is a naturally
recurring state of relatively suspended sensory and motor activity,
characterized by total or partial unconsciousness and the
inactivity of nearly all voluntary muscles. It is distinguished
from quiet wakefulness by a decreased ability to react to stimuli,
and it is more easily reversible than hibernation or coma. It is
observed in all mammals, all birds, and many
reptiles, amphibians, and fish. In
humans, other mammals, and a substantial majority of other animals
that have been studied (such as some species of fish, birds, ants,
and fruit flies), regular sleep is
essential for survival.

The purposes and mechanisms of sleep are only partially clear and
are the subject of intense research.

Sleep cycles through the night, with
deep sleep early on and more REM (marked in red) toward
morning.

Sleep proceeds in cycles of REM and NREM, the order normally being
N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep
(stage N3) early in the night, while the proportion of REM sleep
increases later in the night and just before natural
awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis and his coworkers, who
separated the different EEG features of sleep into five levels (A
to E), which represented the spectrum from wakefulness to deep
sleep. In 1953, REM sleep was discovered as distinct, and thus
William Dement and Nathaniel Kleitman reclassified sleep
into four NREM stages and REM. The staging criteria were
standardized in 1968 by Allan
Rechtschaffen and Anthony Kales in
the "R&K sleep scoring manual." In the R&K standard, NREM
sleep was divided into four stages, with slow-wave sleep comprising
stages 3 and 4. In stage 3, delta waves made up less than 50% of
the total wave patterns, while they made up more than 50% in stage
4. Furthermore, REM sleep was sometimes referred to as stage
5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force
to review the R&K scoring system. The review resulted in
several changes, the most significant being the combination of
stages 3 and 4 into Stage N3. The revised scoring was published in
2007 as The AASM Manual for the Scoring of Sleep and Associated
Events. Arousals and respiratory, cardiac, and movement events
were also added.

Sleep stages and other characteristics of sleep are commonly
assessed by polysomnography in a
specialized sleep laboratory. Measurements taken include electroencephalography (EEG) of brain
waves, electrooculography (EOG)
of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each
sleep cycle lasts from 90 to 110 minutes on average, and each stage
may have a distinct physiological function. Drugs such as sleeping pills and alcoholic beverages can suppress certain
stages of sleep, leading to sleep
deprivation . This can result in sleep that exhibits loss of consciousness but does not fulfill
its physiological functions (i.e., one may still feel tired after
apparently sufficient sleep). REM and slow-wave sleep are both
homeostatically driven; people and most animals selectively
deprived of one of these stages will rebound once uninhibited sleep
is allowed. This finding suggests that both of these stages are
essential.

NREM sleep

According to the 2007 AASM standards, NREM consists of three
stages. There is relatively little dreaming in NREM.

Stage N1 refers to the transition of the brain
from alpha waves having a frequency of 8
to 13 Hz (common in the awake state) to
theta waves having a frequency of 4 to
7 Hz. This stage is sometimes referred to as somnolence or drowsy
sleep. Sudden twitches and hypnic jerks,
also known as positive myoclonus, may be
associated with the onset of sleep during N1. Some people may also
experience hypnagogic
hallucinations during this stage, which can be troublesome to
them. During N1, the subject loses some muscle tone and most conscious awareness of the
external environment.

Stage N2 is characterized by sleep spindles ranging from 11 to 16 Hz (most
commonly 12–14 Hz) and K-complexes. During
this stage, muscular activity as measured by EMG decreases, and
conscious awareness of the external environment disappears. This
stage occupies 45% to 55% of total sleep in adults.

Stage N3 (deep or slow-wave sleep) is characterized by the
presence of a minimum of 20% delta waves
ranging from 0.5 to 2 Hz and having a peak-to-peak amplitude >
75 μV. (EEG standards define delta waves to be from 0 – 4 Hz, but
sleep standards in both the original R&K, as well as the new
2007 AASM guidelines have a range of .5 to 2 Hz.) This is the stage
in which such parasomnias as night terrors, bedwetting, sleepwalking, and sleep-talking occur.

REM sleep

Rapid eye movement sleep, or REM sleep, accounts for 20%–25% of
total sleep time in most human adults. The criteria for REM sleep
include rapid eye movements as well as a rapid low-voltage EEG.
Most memorable dreaming occurs in this stage. At least in mammals,
a descending muscular atonia is seen. Such
paralysis may be necessary to protect organisms from self-damage
through physically acting out scenes from the often-vivid dreams
that occur during this stage.

Timing

The human biological clock

Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain
bounds, willed behavior. The circadian clock — an inner
timekeeping, temperature-fluctuating, enzyme-controlling device —
works in tandem with adenosine, a
neurotransmitter that inhibits many of the bodily processes
associated with wakefulness. Adenosine is created over the course
of the day; high levels of adenosine lead to sleepiness. In
diurnal animals, sleepiness occurs as the
circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by
one's chronotype. It is the circadian
rhythm that determines the ideal timing of a correctly structured
and restorative sleep episode.

Homeostatic sleep propensity (the need for sleep as a function of
the amount of time elapsed since the last adequate sleep episode)
must be balanced against the circadian element for satisfactory
sleep. Along with corresponding messages from the circadian clock,
this tells the body it needs to sleep. Sleep offset (awakening) is
primarily determined by circadian rhythm. A person who regularly
awakens at an early hour will generally not be able to sleep much
later than their normal waking time, even if moderately
sleep-deprived.

Sleep duration is affected by circadian
rhythm which is regulated by a gene named DEC2. Some people have a mutation of this gene; they
sleep two hours less than normal. Neurology professor Ying-Hui Fu
and his colleagues bred mice that carried the DEC2 mutation and
slept less than normal mice.

Optimal amount in humans

Adult

The optimal amount of sleep is not a meaningful concept unless the
timing of that sleep is seen in relation to an individual's
circadian rhythms. A person's major
sleep episode is relatively inefficient and inadequate when it
occurs at the "wrong" time of day; one should be asleep at least
six hours before the lowest body temperature. The timing is correct
when the following two circadian markers occur after the middle of
the sleep episode and before awakening:

maximum concentration of the hormone melatonin, and

minimum core body temperature.

The National Sleep
Foundation in the United States maintains that seven to nine
hours of sleep for adult humans is optimal and that sufficient
sleep benefits alertness, memory, problem solving, and overall health, as well
as reducing the risk of accidents. A widely publicized 2003 study performed
at the University
of Pennsylvania School of Medicine demonstrated that cognitive
performance declines with six or fewer hours of sleep.

A University of
California, San Diego, psychiatry study of more than one million adults
found that people who live the longest self-report sleeping for six
to seven hours each night. Another study of sleep duration
and mortality risk in women showed similar results. Other studies
show that "sleeping more than 7 to 8 hours per day has been
consistently associated with increased mortality," though this
study suggests the cause is probably other factors such as
depression and socioeconomic status, which would correlate
statistically. It has been suggested that the correlation between
lower sleep hours and reduced morbidity only occurs with those who
wake after less sleep naturally, rather than those who use an
alarm.

Researchers at the University of
Warwick and University College London have found that lack of sleep can more than double
the risk of death from cardiovascular disease, but that too
much sleep can also be associated with a doubling of the risk of
death, though not primarily from cardiovascular disease.
Professor Francesco Cappuccio said, "Short sleep has been shown to
be a risk factor for weight gain,
hypertension, and Type 2 diabetes, sometimes leading
to mortality; but in contrast to the short sleep-mortality
association, it appears that no potential mechanisms by which long
sleep could be associated with increased mortality have yet been
investigated. Some candidate causes for this include depression,
low socioeconomic status, and cancer-related fatigue. …In terms of
prevention, our findings indicate that consistently sleeping around
seven hours per night is optimal for health, and a sustained
reduction may predispose to ill health."

Furthermore, sleep difficulties are closely associated with
psychiatric disorders such as depression, alcoholism, and bipolar disorder. Up to 90% of adults with
depression are found to have sleep difficulties. Dysregulation
found on EEG includes disturbances in sleep continuity, decreased
delta sleep and altered REM patterns with regard to latency,
distribution across the night and density of eye movements.

Hours by age

Children need more sleep per day in order to develop and function
properly: up to 18 hours for newborn babies,
with a declining rate as a child ages. A newborn baby spends almost
9 hours a day in REM sleep. By the age of five or so, only slightly
over two hours is spent in REM.

Age and condition

Average amount of sleep per day

Newborn

up to 18 hours

1–12 months

14–18 hours

1–3 years

12–15 hours

3–5 years

11–13 hours

5–12 years

9–11 hours

Adolescents

9–10 hours

Adults, including elderly

7–8 (+) hours

Pregnant women

8 (+) hours

Sleep debt

Sleep debt is the effect of not getting enough rest and sleep; a
large debt causes mental, emotional, and physical fatigue. It is
unclear why a lack of sleep causes irritability; however, theories
are emerging that suggest if the body produces insufficient
cortisol during deep sleep, it can have
negative effects on the alertness and emotions of a person during
the day.

Scientists do not agree on how much sleep debt it is possible to
accumulate; whether it is accumulated against an individual's
average sleep or some other benchmark; nor on whether the
prevalence of sleep debt among adults has changed appreciably in
the industrialized world in recent
decades. It is likely that children are sleeping less than
previously in Western societies.

Genetics

A considerable amount of sleep-related behavior is apparently
hard-wired into human biology - humans in all cultures get tired,
require sleep for good health, and have similar symptoms when sleep
deprived. Scientific research has identified some genetic
variations, including:

A mutation that moves consolidated sleep earlier, resulting in
a sleep cycle from 7:30pm to 3:30am.

A mutation in BHLHB3 which apparently
reduces the amount sleep needed for healthy living to 6 hours from
8.

Functions

The multiple theories proposed to explain the function of sleep
reflect the as-yet incomplete understanding of the subject. It is
likely that sleep evolved to fulfill some primeval function and has
taken over multiple functions over time. (As an analogy, the
larynx in all mammals controls the passage of
food and air, but may have descended in humans to take on
additional language capabilities.)

It has been pointed out that, if sleep were not essential, one
would expect to find 1) animal species that do not sleep at all, 2)
animals that do not need recovery sleep when they stay awake longer
than usual, and 3) animals that suffer no serious consequences as a
result of lack of sleep. No animals have been found to date that
satisfy any of these criteria.

Some of the many proposed functions of sleep are as follows.

Restoration

Wound healing has been shown to be
affected by sleep. A study conducted by Gumustekin et al.
in 2004 shows sleep deprivation hindering the healing of burns on rats.

It has been shown that sleep deprivation affects the immune system. In a study by Zager et al. in
2007, rats were deprived of sleep for 24 hours. When compared with
a control group, the
sleep-deprived rats' blood tests
indicated a 20% decrease in white blood
cell count, a significant change in the immune system. It is
now possible to state that "sleep loss impairs immune function and
immune challenge alters sleep," and it has been suggested that
mammalian species which invest in longer sleep times are investing
in the immune system, as species with the longer sleep times have
higher white blood cell counts.

It has yet to be proven that sleep duration affects somatic growth. One study by Jenni et al. in 2007
recorded growth, height, and weight, as correlated to
parent-reported time in bed in 305 children over a period of nine
years (age 1–10). It was found that "the variation of sleep
duration among children does not seem to have an effect on growth."
It has been shown that sleep—more specifically, slow-wave sleep
(SWS)—does affect growth hormone
levels in adult men. During eight hours' sleep, Van Cauter,
Leproult, and Plat found that the men with a high percentage of SWS
(average 24%) also had high growth hormone secretion, while
subjects with a low percentage of SWS (average 9%) had low growth
hormone secretion.

There are multiple arguments supporting the restorative function of
sleep. We are rested after sleeping, and it is natural to assume
that this is a basic purpose of sleep. The metabolic phase during
sleep is anabolic; anabolic hormones such as growth hormones (as
mentioned above) are secreted preferentially during sleep. The
duration of sleep among species is, in general, inversely related to animal size and
directly related to basal metabolic
rate. Rats with a very high basal metabolic rate sleep for up
to 14 hours a day, whereas elephants and giraffes with lower BMRs
sleep only 3–4 hours per day.

Energy conservation could as well have been accomplished by resting
quiescent without shutting off the organism from the environment,
potentially a dangerous situation. A sedentary nonsleeping animal
is more likely to survive predators, while still preserving energy.
Sleep, therefore, seems to serve another purpose, or other
purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation
go into rebound sleep because of lack of sleep during the
hibernation period. They are definitely well-rested and are
conserving energy during hibernation, but need sleep for something
else. Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually, septicemia and death.

Anabolic/catabolic

Non-REM sleep may be an anabolic state marked by physiological processes
of growth and rejuvenation of the organism's immune, nervous,
muscular, and skeletal systems (with some exceptions). Wakefulness
may perhaps be viewed as a cyclical, temporary, hyperactive
catabolic state during which the organism
acquires nourishment and reproduces.

Ontogenesis

According to the ontogenetic hypothesis of
REM sleep, the activity occurring during neonatal REM sleep (or
active sleep) seems to be particularly important to the developing
organism (Marks et al., 1995). Studies investigating the effects of
deprivation of active sleep have shown that deprivation early in
life can result in behavioral problems, permanent sleep disruption,
decreased brain mass (Mirmiran et al., 1983), and an abnormal
amount of neuronal cell death (Morrissey, Duntley & Anch,
2004).

REM sleep appears to be important for development of the brain. REM
sleep occupies the majority of time of sleep of infants, who spend
most of their time sleeping. Among different species, the more
immature the baby is born, the more time it spends in REM sleep.
Proponents also suggest that REM-induced muscle inhibition in the
presence of brain activation exists to allow for brain development
by activating the synapses, yet without any motor consequences that
may get the infant in trouble. Additionally, REM deprivation
results in developmental abnormalities later in life.

However, this does not explain why older adults still need REM
sleep. Aquatic mammal infants do not
have REM sleep in infancy; REM sleep in those animals increases as
they age.

Memory processing

Scientists have shown numerous ways in which sleep is related to
memory. In a study conducted by Turner,
Drummond, Salamat, and Brown, working
memory was shown to be affected by sleep deprivation. Working
memory is important because it keeps information active for further
processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic
memory. The study allowed 18 women and 22 men to sleep only 26
minutes per night over a four-day period. Subjects were given
initial cognitive tests while
well-rested, and then were tested again twice a day during the four
days of sleep deprivation. On the final test, the average working
memory span of the sleep-deprived group had dropped by 38% in
comparison to the control group.

Memory seems to be affected differently by certain stages of sleep
such as REM and slow-wave sleep
(SWS). In one study cited in Born, Rasch, and Gais, multiple groups
of human subjects were used: wake control groups and sleep test
groups. Sleep and wake groups were taught a task and were then
tested on it, both on early and late nights, with the order of
nights balanced across participants. When the subjects' brains were
scanned during sleep, hypnograms revealed that SWS was the dominant
sleep stage during the early night, representing around 23% on
average for sleep stage activity. The early-night test group
performed 16% better on the declarative memory test than the control
group. During late-night sleep, REM became the most active sleep
stage at about 24%, and the late-night test group performed 25%
better on the procedural memory
test than the control group. This indicates that procedural memory
benefits from late, REM-rich sleep, whereas declarative memory
benefits from early, SWS-rich sleep.

A study conducted by Datta indirectly supports these results. The
subjects chosen were 22 male rats. A box was constructed wherein a
single rat could move freely from one end to the other. The bottom
of the box was made of a steel grate. A light would shine in the
box accompanied by a sound. After a five-second delay, an
electrical shock would be applied. Once the shock commenced, the
rat could move to the other end of the box, ending the shock
immediately. The rat could also use the five-second delay to move
to the other end of the box and avoid the shock entirely. The
length of the shock never exceeded five seconds. This was repeated
30 times for half the rats. The other half, the control group, was
placed in the same trial, but the rats were shocked regardless of
their reaction. After each of the training sessions, the rat would
be placed in a recording cage for six hours of polygraphic
recordings. This process was repeated for three consecutive days.
This study found that during the posttrial sleep recording session,
rats spent 25.47% more time in REM sleep after learning trials than
after control trials. These trials support the results of the Born
et al. study, indicating an obvious correlation between REM sleep
and procedural knowledge.

An observation of the Datta study is that the learning group spent
180% more time in SWS than did the control group during the
post-trial sleep-recording session. This phenomenon is supported by
a study performed by Kudrimoti, Barnes, and McNaughton. This study
shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during
SWS following the experiment. In a study by Kudrimoti et al., seven
rats were run through a linear track using rewards on either end.
The rats would then be placed in the track for 30 minutes to allow
them to adjust (PRE), then they ran the track with reward-based
training for 30 minutes (RUN), and then they were allowed to rest
for 30 minutes. During each of these three periods, EEG data were collected for
information on the rats' sleep stages. Kudrimoti et al. computed
the mean firing rates of hippocampal place cells during prebehavior
SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST)
by averaging across 22 track-running sessions from seven rats. The
results showed that ten minutes after the trial RUN session, there
was a 12% increase in the mean firing rate of hippocampal place
cells from the PRE level; however, after 20 minutes, the mean
firing rate returned rapidly toward the PRE level. The elevated
firing of hippocampal place cells during SWS after spatial
exploration could explain why there were elevated levels of SWS
sleep in Datta's study, as it also dealt with a form of spatial
exploration.

The different studies all suggest that there is a correlation
between sleep and the complex functions of memory. Harvard sleep
researchers Saper and Stickgold point out that an essential part of
memory and learning consists of nerve cell dendrites' sending information to the cell body to
be organized into new neuronal connections. This process demands
that no external information is presented to these dendrites, and
they suggest that this may be why it is during sleep that we
solidify memories and organize knowledge.

Preservation

The "Preservation and Protection" theory holds that sleep serves an
adaptive function. It protects the animal during that portion of
the 24-hour day in which being awake, and hence roaming around,
would place the individual at greatest risk. Organisms do not
require 24 hours to feed themselves and meet other necessities.
From this perspective of adaptation, organisms are safer by staying
out of harm's way, where potentially they could be prey to other,
stronger organisms. They sleep at times that maximize their safety,
given their physical capacities and their habitats. (Allison &
Cicchetti, 1976; Webb, 1982).

However, this theory fails to explain why the brain disengages from
the external environment during normal sleep. Another argument
against the theory is that sleep is not simply a passive
consequence of removing the animal from the environment, but is a
"drive"; animals alter their behaviors in order to obtain sleep.
Therefore, circadian regulation is more than sufficient to explain
periods of activity and quiescence that are adaptive to an
organism, but the more peculiar specializations of sleep probably
serve different and unknown functions. Moreover, the preservation
theory does not explain why carnivores like lions, which are on top
of the food chain, sleep the most. By the
preservation logic, these top carnivores should not need any sleep
at all.

Preservation also does not explain why aquatic mammals sleep while
moving. Quiescence during these vulnerable hours would do the same
and would be more advantageous, because the animal would still be
able to respond to environmental challenges like predators, etc.
Sleep rebound that occurs after a sleepless night will be
maladaptive, but obviously must occur for a reason. A zebra falling
asleep the day after it spent the sleeping time running from a lion
is more, not less, vulnerable to predation.

Dreaming

Dreaming is the perception of sensory images and sounds during
sleep, in a sequence which the dreamer usually perceives more as an
apparent participant than an observer. Dreaming is stimulated by
the pons and mostly occurs during the REM phase of sleep.

Freud's work concerns the psychological role of dreams, which
clearly does not exclude any physiological role they may have. It
is not ruled out therefore by the increased modern interest in the
organization and consolidation of recent memory and experience. Recent research claims that sleep has this overall
role of consolidation and organization of synaptic connections
formed during learning and experience.

Effect of food and drink on sleep

Sedatives

Nonbenzodiazepine hypnotics,
better known as sleep aids, such as eszopiclone (Lunesta), zalepon , and zolpidem are
commonly used as sleep aids prescribed by doctors to treat forms of
insomnia. Nonbenzodiazopines are the most commonly prescribed and
OTC sleep aids used worldwide and have been
greatly growing in use since the 1990s. They target the GABAA receptor.

Depressants

Often, people start drinking alcohol in order to get to sleep
(alcohol is initially a sedative and will cause somnolence, encouraging sleep). However, being
addicted to alcohol can lead to disrupted sleep, because alcohol
has a rebound effect later in the
night. As a result, there is strong evidence linking alcoholism and
insomnia.

Melatonin is a naturally occurring hormone that regulates
sleepiness. It is made in the brain, where tryptophan is converted
into serotonin and then into melatonin, which is released at night
by the pineal gland to induce and
maintain sleep. Melatonin supplementation may be used as a sleep
aid, both as a hypnotic and as a chronobiotic (see phase response curve, PRC).

Many people have a temporary drop in alertness in the early
afternoon, commonly known as the "post-lunch dip." While a large
meal can make a person feel sleepy, the post-lunch dip is mostly an
effect of the biological clock.
People naturally feel most sleepy (have the greatest "drive for
sleep") at two times of the day about 12 hours apart—for example,
at 2:00 a.m. and 2:00 p.m. At those two times, the body clock
"kicks in." At about 2 p.m. (14:00), it overrides the homeostatic
buildup of sleep debt, allowing several more hours of wakefulness.
At about 2 a.m. (02:00), with the daily sleep debt paid off, it
"kicks in" again to ensure a few more hours of sleep.

The amino acid tryptophan is a building block of proteins. It has
been claimed to contribute to sleepiness, since it is a precursor
of the neurotransmitter serotonin, involved in sleep regulation.
However, no solid data have ever linked modest dietary changes in
tryptophan to changes in sleep.

Stimulants

Amphetamines (amphetamine, dextroamphetamine, methamphetamine, etc.) are often used to
treat narcolepsy and ADHD disorders and when used recreationally may be
referred to as "speed." Their most common effects are anxiety,
insomnia, stimulation, increased alertness, and decreased hunger.
Adderall is a mixture of amphetamine salts
used to treat ADHD.

Caffeine is a stimulant that works by
slowing the action of the hormones in the brain that cause somnolence, particularly by acting as an antagonist at adenosine
receptors. Effective dosage is individual, in part dependent on
prior usage. It can cause a rapid reduction in alertness as it
wears off.

Studies on cocaine have shown its effects to be mediated through
the circadian rhythm system. This may be related to the onset of
hypersomnia (oversleeping) in regard to
"Cocaine-Induced Sleep Disorder."

The stimulating effects of energy drinks come from stimulants such
as caffeine, sugars, and essential amino acids, and they will
eventually create a rapid reduction in alertness similar to that of
caffeine.

Older people are more easily awoken by
disturbances in the environment and may to some degree lose the
ability to consolidate sleep. They need the same amount per day as
they've always needed, but may need to take some of their sleep as
daytime naps.

Anthropology of sleep

Research suggests that sleep patterns vary significantly across
cultures. The most striking differences are between societies that
have plentiful sources of artificial light and ones that do not.
The primary difference appears to be that prelight cultures have
more broken-up sleep patterns. For example, people might go to
sleep far sooner after the sun sets, but then wake up several times
throughout the night, punctuating their sleep with periods of
wakefulness, perhaps lasting several hours. The boundaries between
sleeping and waking are blurred in these societies. Some observers
believe that nighttime sleep in these societies is most often split
into two main periods, the first characterised primarily by deep
sleep and the second by REM sleep. This segmented sleep has led to expressions such
as "first sleep," "watch," and "second sleep," which appear in
literature from preindustrial
societies all over the world.

Some societies display a fragmented sleep pattern in which people
sleep at all times of the day and night for shorter periods. In
many nomadic or hunter-gatherer societies, people will sleep
on and off throughout the day or night depending on what is
happening.Plentiful artificial
light has been available in the industrialised West since at
least the mid-19th century, and sleep patterns have changed
significantly everywhere that lighting has been introduced In
general, people sleep in a more concentrated burst through the
night, going to sleep much later, although this is not always
true.

In some societies, people generally sleep with at least one other
person (sometimes many) or with animals. In other cultures, people
rarely sleep with anyone but a most intimate relation, such as a
spouse. In almost all societies, sleeping partners are strongly
regulated by social standards. For example, people might only sleep
with their immediate family,
extended family, spouses, their
children, children of a certain age, children of specific gender,
peers of a certain gender, friends, peers of equal social rank, or
with no one at all. Sleep may be an actively social time, depending
on the sleep groupings, with no constraints on noise or
activity.

People sleep in a variety of locations. Some sleep directly on the
ground; others on a skin or blanket; others sleep on platforms or
beds. Some sleep with blankets, some with
pillows, some with simple headrests, some with no head support.
These choices are shaped by a variety of factors, such as climate,
protection from predators, housing type, technology, and the
incidence of pests.

Sleep in nonhumans

Many animals sleep, but neurological sleep states are difficult to
define in lower-order animals. In these animals, sleep is defined
using behavioral characteristics such as minimal movement, postures
typical for the species, and reduced responsiveness to external
stimulation. Sleep is quickly reversible, as opposed to hibernation
or coma, and sleep deprivation is followed by
longer or deeper sleep. Herbivores, who require a long waking
period to gather and consume their diet, typically sleep less each
day than similarly sized carnivores, who might well consume several
days' supply of meat in a sitting.

Horses and other herbivorous ungulates can
sleep while standing, but must necessarily lie down for REM sleep
(which causes muscular atony) for short periods. Giraffes, for
example, only need to lie down for REM sleep for a few minutes at a
time. Bats sleep while hanging upside down. Some aquatic mammals
and some birds can sleep with one half of the brain while the other
half is awake, so-called unihemispheric slow-wave
sleep. Birds and mammals have cycles of non-REM and REM sleep
(as described above for humans), though birds’ cycles are much
shorter and they do not lose muscle tone (go limp) to the extent
that most mammals do.

Many mammals sleep for a large proportion of each 24-hour period
when they are very young. However, killer
whales and some dolphins do not sleep
during the first month of life. Such differences may be explained
by the ability of land-mammal newborns to be easily protected by
parents while sleeping, while marine animals must, even while very
young, be more continuously vigilant for predators.